Isn't the amount of biowaste exported related to the population?
Yes and no.
The amount of any commodity imported or exported "at rest" before people start buying or selling it depends on three factors:
1) The economic size of the station. At a "normal" station this is approximately equal to the square root of the population, but can be hand-edited independently by Frontier. The Colonia region's economic size is - taking bubble stations as a neutral baseline - about 4 times larger than its population.
2) The current Political states of the station controller.
https://cdb.sotl.org.uk/effects has what I've been able to find out there - they affect both pricing and "at rest" quantities, in roughly the ways you'd expect from a narrative consideration of what the state is.
3) A station-specific random factor so that not every Refinery is a clone of every other Refinery. Known ranges are at
https://cdb.sotl.org.uk/specialisation - the units are arbitrary so that the economic size can be taken as sqrt(population) without having to apply an additional scale factor
I use Hydrogen Fuel rather than Biowaste for establishing economy sizes for a few reasons:
1) There's even less reason to trade it, so you're more likely to see a true "at rest" figure in the market without needing lengthy observations (though Biowaste is generally fine for this too)
2) Everywhere exports it so you don't need a separate rule for Agricultural or Agricultural-hybrid economies (you do - below - need a separate rule for Colony)
3) Its specialisation range is tiny (I treat it as constant, for simplicity, but that's probably not quite true nowadays) so long as you remember that Colony economies have a different number - whereas Biowaste has a very substantial variation.
There's a bit more complexity around hybrid economies though that doesn't tend to matter for H-Fuel or Biowaste. HE Suits are a good example of this - there are a lot of High-Tech/Extraction or High-Tech/Refinery stations in the bubble, which are 80% HT and 20% the other. HT produces, Extraction or Refinery consumes, but if you look at the specialisation page, the average consumption is about 8 times the average production for the same economy size - so with the station hybridisation only being 4:1 the other way, it's incredibly rare for those stations to actually have HE Suits available to sell.